1. Field of Invention
This invention relates generally to an electrostatic chuck used for holding a semiconductor wafer during processing and, in particular, to such a chuck that utilizes ceramic for protection of its pole pieces.
2. Description of the Prior Art
It is known to use thick semiconductive ceramic coatings such as alumina/titania on the clamping surface of an electrostatic chuck using thermal spray techniques. These coatings have a large difference in thermal expansion from the chuck body, do not break down destructively and must have a narrow range of resistivity to operate successfully. Also, the thickness of these coatings leads to excessive thermal resistance to the coated metal chuck body. These coatings also draw current detrimentally from a treating plasma environment when the coatings are carried out to the edge of the clamping surface, where contact with the plasma may take place. This material cannot be used in the joints between clamping poles and a frame member, as there will be too much conductive current drawn to the frame member.
It is known also to use insulating ceramic coatings such as alumina by applying same to the metallic clamping surface using thermal spray techniques. The application of such coatings to the body of the metallic chuck without stress failure is difficult. Breakdown field strength is not much different than the best anodized aluminum coatings. For bipolar or multipolar chucks, the ceramic must be applied to each pole and appropriate insulation applied for isolation of each pole from the support structure. If thick ceramic is used, this usually results in excessive thermal resistance between the pole and the coated support structure. Also, thick insulative coatings require very high clamping voltages (several thousand) which make the isolation of connections much more difficult to achieve.
It is known also to use organic coatings such as polyimide (Kapton) over a metal chuck. Such coatings have very high breakdown strength, but are easily damaged and are attacked easily by process plasmas.
Thick crystallization insulative coatings, such as sapphire, have been known to be bonded to the chuck clamping surface, usually in thick sheets. Such coatings have very high breakdown field strength, but are very brittle, difficult to apply to large areas in thin sheets and are very expensive.
It is also known to fire ceramic sheets with embedded metallic layers to form a chuck assembly and then bond same to a metal cooling/heating assembly containing connections to the chuck's electrodes.
The U.S. Pat. No. 4,480,284 to Tojo discloses an electrostatic chuck plate consisting of a flat electrode plate, a dielectric layer formed on one flat surface of the electrode plate by flame spraying and impregnated with plastics.
The U.S. Pat. No. 5,055,964 to Logan, et al discloses an electrostatic chuck on which an epoxy bond is formed between the anodized electrodes of the chuck.
The U.S. Pat. No. 5,151,845 to Watanabe, et al, discloses an electrostatic chuck with multiple insulating films of different insulation resistances.
The U.S. Pat. No. 5,463,526 to Mundt discloses an electrostatic chuck with a conducting layer covered with a thick semi conducting layer which in turn is covered with a thin dielectric layer upon which a wafer to be held and treated is placed.
In Barnes, et al, U.S. Pat. No. 5,467,249 an electrostatic chuck is disclosed wherein the outer electrode is surrounded by a dielectric ring having a dielectric inner radius less than the workpiece.
In U.S. Pat. No. 5,535,507 to Barnes, et al, a method of forming an electrostatic chuck is described in which a polyimide film is inserted between the anodized surfaces of the chuck's electrode. The present invention represents a specific improvement over Barnes, et al.
The U.S. Pat. No. to Wu, et al, 5,560,780 discloses an electrostatic chuck having a polymeric dielectric material (polyimide) formed thereon and protected by an inorganic aluminum compound such as aluminum oxide or aluminum nitride.
The U.S. Pat. No. 5,691,876 Chen, et al., to utilizes a polymide film within an electrostatic chuck as a dielectric layer.
What the prior art lacks is the ability to design and build monopolar, bipolar or multipolar chucks using ceramic semiconductive materials over low-cost aluminum base materials, provide insulative protection at the perimeter of the chuck to prevent harmful contact of the semiconductive ceramic materials with the surrounding environment (usually a plasma), and use anodized aluminum as a dielectric material to internally isolate the pole pieces or coat other features of the chuck such as the water channels.